A. Field of the Invention
The present invention relates generally to optical couplers, and, more particularly to an optical coupler for coupling an optical fiber into a silicon-on-insulator (SOI) waveguide.
B. Description of the Related Art
Silicon-on-insulator (SOI) is one of the more recent and promising integrated optics technologies because it allows the use of conventional microelectronics patterning and fabrication techniques and it offers a high index contrast for strong light confinement in small dimensions. This enables the miniaturization of functional integrated optical devices and the ability to monolithically integrate electronic and optical circuits. For nanometer SOI integrated optics, one of the key issues is efficient light coupling into SOI waveguides. Following the conventional methods for coupling into polymer waveguides, four main approaches are taken to couple light into SOI waveguides: transverse coupling, grating coupling, prism coupling, and vertical tapering.
Transverse coupling, including end-fire coupling and end-butt coupling, has been a coupling efficiency approximately proportional to the mode dimension ratio. However, the mode size of a single mode SOI waveguide is often on the order of hundreds of nanometers due to higher refractive index contrast. As a result, very low efficiency occurs for transverse coupling.
Grating coupling is not well-suited for optical integrated circuits (OIC) due to difficulty in mode matching, sensitivity to wavelengths, complexity of design and fabrication. One solution to these problems includes a 90 degree grating coupler designed and fabricated from an optical fiber to wide waveguide. Two-dimensional simulations show that up to a 74% coupling efficiency between a single-mode optical fiber and a 240-nm-thick GaAs—AlOx waveguide may be achieved, however, the coupling efficiency was measured to be only 19%. Another proposed solution includes a fiber-waveguide coupler composed of two one to three millimeter-long gratings and four layers. A 95% coupling efficiency can be achieved with this, however, the structure is difficult to fabricate.
Thus, there is a need in the art for an optical coupler for coupling an optical fiber to a SOI waveguide that overcomes the problems of the related art.